Millikan Investigates Cosmic Rays
Robert Andrews Millikan's investigations into cosmic rays were pivotal in understanding this mysterious form of radiation that originates from outer space. The concept of cosmic rays began to take shape in the early 20th century when physicist Victor Franz Hess conducted balloon experiments and discovered that radiation increased with altitude, suggesting an extraterrestrial source. Millikan, an American physicist, sought to clarify the nature of these rays through his own experiments conducted in the 1920s. His notable experiments involved measuring the penetrating power of cosmic rays in locations with significant elevation, leading him to conclude that these rays were indeed cosmic in origin.
In his findings, Millikan labeled this radiation as "cosmic rays," hypothesizing that they were primarily composed of high-energy photons. However, subsequent research, particularly by physicists like Walther Bothe and Arthur Holly Compton, challenged Millikan's conclusions by demonstrating that cosmic rays are composed of charged particles, influenced by Earth's magnetic field. This evolving understanding of cosmic rays reflects the collaborative and contested nature of scientific inquiry, illustrating how new evidence can reshape established theories and lead to a deeper comprehension of the universe.
Millikan Investigates Cosmic Rays
Date 1920-1930
Robert Andrews Millikan and his colleagues proved that upper-atmospheric radiation was of extraterrestrial origin, named it, and led in its exploration.
Locale United States
Key Figures
Robert Andrews Millikan (1868-1953), American physicistVictor Franz Hess (1883-1964), Austrian physicistArthur Holly Compton (1892-1962), American physicistWerner Kolhörster (1887-1946), German physicist
Summary of Event
Cosmic radiation was discovered by the Swiss physicist Albert Gockel in 1910. It was the Austrian physicist Victor Franz Hess, however, who first proposed that the radiation was of extraterrestrial origin. Hess had been working on the problem of air ionization, a phenomenon known since the beginning of the twentieth century. It was initially supposed that this slight ionization was caused by radioactive elements in the earth. This meant that at greater altitudes the radiation should decrease. Yet in 1911, when Hess ascended in a balloon to 5,200 meters (about 17,060 feet), he found that the radiation increased above 2,000 meters (6,562 feet) and that above 3,000 meters (9,843 feet) there was an even sharper rise in intensity. Hess concluded that the “penetrating radiation,” as it was then known, entered the atmosphere from above. Hess’s work was confirmed in 1913 by Werner Kolhörster, a German physicist who made a balloon ascent to 9,000 meters (29,528 feet).
Not all physicists believed the explanation Hess and Kolhörster gave, however, and even the very existence of the radiation remained in some doubt. The idea that a strong source of radiation existed in outer space was simply unimaginable to some scientists. Some believed that radioactive uranium and thorium in the soil caused the radiation. Others believed that the equipment used by Hess was flawed. Still others sought a compromise position, believing that the radiation was produced somewhere higher up in Earth’s atmosphere, something that Hess admitted was a possibility. Robert Andrews Millikan, an American physicist, decided to try to settle the matter.
Millikan’s first experiments were carried out in 1921-1922 and involved sending a number of sounding balloons into the atmosphere. These experiments were inconclusive. More tests were performed in 1922 and 1923 and were similarly inconclusive. Then, in the summer of 1925, Millikan designed an experiment to determine the penetrating power of the rays, which had not yet been measured. If the rays were of external origin, they would have to have a penetrating power great enough to get through the atmosphere, which was equivalent to penetrating 10 meters (32.8 feet) of water. The strongest rays produced by known radioactive elements could not penetrate more than about 2 meters (6.56 feet) of water.
With his assistant, George Harvey Cameron, Millikan took measurements from two California lakes, Muir Lake (elevation 3,650 meters, or 11,975 feet), near Mount Whitney, and Lake Arrowhead (elevation 1,500 meters, or 4,921 feet). At Muir Lake, Millikan and Cameron lowered their electroscopes into the water. They found that the radiation, coming exclusively from above, was eighteen times greater than that of the strongest known gamma ray (gamma rays are the strongest of the three types of radioactive emissions). This radiation was strong enough to penetrate the atmosphere, proving that the radiation certainly could come from space.
At Lake Arrowhead, Millikan and Cameron found that the readings were identical to the Muir Lake readings when the difference in lake elevation was taken into account. This proved to Millikan that the atmosphere played no part in transmitting the rays but acted merely as an absorbing medium. Millikan now believed that the rays came from outer space.
In late 1925, at a meeting of the National Academy of Sciences, Millikan announced his findings, calling the new radiation “cosmic rays.” He believed that, as the strongest radiation previously known was that produced in radioactive transformations, these cosmic rays were the result also of some sort of nuclear charge. The strongest rays on Earth were photons (particles of electromagnetic radiation), which were produced when helium was produced from hydrogen atoms and when an electron was captured by a light nucleus. Millikan inferred, therefore, that cosmic rays were photons produced from some type of atom formation.
Millikan next tested his assumption that cosmic rays were composed of high-energy photons. In a 1926 trip to Lake Titicaca in South America, he noticed almost no difference between the findings there and at Muir Lake. If cosmic rays had instead been composed of charged particles, Earth’s magnetic field would have affected the radiation distribution across the globe. Measurements on the return boat trip from Peru to Los Angeles also showed no variation.
Significance
The notion that photons were the primary constituents of cosmic rays was challenged in 1929. Kolhörster and German physicist Walther Bothe , after a series of experiments with a Geiger-Müller counter, concluded that cosmic rays were, in fact, composed of charged particles. Following this work, many physicists turned to the problem of which model was correct.
The decisive experiment was made by Nobel laureate Arthur Holly Compton. He turned again to the question of what effect, if any, Earth’s magnetic field had on the intensity of cosmic rays. Even though Millikan, and later Kolhörster, failed to notice any appreciable difference, a growing body of work, beginning in 1927, pointed toward a “latitude effect.” In 1932, Compton organized a massive survey of the globe, trying to detect such an effect. By September, 1932, the results of the survey showed that there was, indeed, a latitude effect, and thus that cosmic rays were composed at least partly of charged particles, a fact that Millikan was forced to accept.
Bibliography
DeVorkin, David H. Race to the Stratosphere: Manned Scientific Ballooning in America. New York: Springer-Verlag, 1989. Excellent history of manned scientific ballooning by the curator of the National Air and Space Museum. Focuses on Auguste and Jean Piccard, with extensive coverage of the cosmic-ray experiments conducted with Millikan and Compton. Scholarly, but very readable. Includes photographs, bibliography, notes, and index.
Friedlander, Michael W. Cosmic Rays. Cambridge, Mass.: Harvard University Press, 1989. Popular account by a practicing researcher includes a good bit of history in discussing all aspects of cosmic-ray research. Features photographs, illustrations, bibliography, and index.
Grieder, Peter K. F. Cosmic Rays at Earth: Researcher’s Reference Manual and Data Book. New York: Elsevier Science, 2001. Reference manual intended for use by scientists involved in cosmic-ray research includes background material that may be useful to lay readers with some science background.
Kargon, Robert H. The Rise of Robert Millikan: Portrait of a Life in American Science. Ithaca, N.Y.: Cornell University Press, 1982. Selective scholarly biography by a historian of science focuses on themes in American science as illuminated by Millikan’s life. Provides much material on the public side of cosmic-ray research. Includes some photographs, notes, and index.
Kevles, Daniel J. The Physicists: The History of a Scientific Community in Modern America. 1977. Reprint. Cambridge, Mass.: Harvard University Press, 2005. Scholarly but accessible work by a historian of science tells the story of the development of the physics community in the United States from its origins through the development of supercolliders. Contains some material on the Millikan-Compton controversy and places the field in its larger context. Includes extensive bibliographic essay, notes, and index.
Millikan, Robert Andrews. Electrons (+ and –), Protons, Photons, Neutrons, Mesotrons, and Cosmic Rays. Rev. ed. Chicago: University of Chicago Press, 1947. All but the last five chapters of this edition are largely unrevised from the original 1937 edition. Presents much of what was known about particle physics at the time; however, the sections on cosmic rays are extremely self-justifying and have been recognized as such by other physicists. Should be read in conjunction with the Kargon book cited above. Includes photographs, illustrations, and index.
Motz, Lloyd, and Jefferson Hane Weaver. The Story of Astronomy. New York: Plenum, 1995. Presents the history of astronomy from ancient times to the end of the twentieth century. Chapter 18 includes discussion cosmic rays. Features bibliography and index.
Pomerantz, Martin A. Cosmic Rays. New York: Van Nostrand Reinhold, 1971. Semipopular account aimed at college undergraduates with some science or mathematics background. Includes a historical chapter that is more accessible to nonscientists. Covers all aspects of research. Features illustrations, photographs, and index.
Rossi, Bruno. Cosmic Rays. New York: McGraw-Hill, 1964. Popular account by a pioneer researcher offers a good introduction to the subject. Includes photographs, illustrations, and index.
Segrè, Emilio. From X-Rays to Quarks: Modern Physicists and Their Discoveries. San Francisco: W. H. Freeman, 1980. Impressionistic account of the development of physics by a practitioner. Includes some material on cosmic rays. Features photographs, annotated bibliography, and index.
Sekido, Yataro, and Harry Elliot, eds. Early History of Cosmic Ray Studies: Personal Reminiscences with Old Photographs. Boston: D. Reidel, 1985. Collection of papers by a number of early cosmic-ray researchers. Especially good on Hess and his predecessors, weak on Millikan. Includes photographs, illustrations, notes, and index.